Conventionally, a spark plug for internal combustion engines for automobiles, etc. contains a sintered ceramic body called "insulator" as a member thereof. The sintered ceramic body is prepared using ceramic powder of alumina or the like, a sintering promoter of silicon oxide (SiO.sub.2), calcium oxide (CaO) or magnesium oxide (MgO), or the like, and an organic binder such as polyvinyl alcohol (PVA). The thus prepared sintered ceramic body is required to have excellent voltage withstanding ability, insulation property and mechanical strength when it is used for spark plugs.
However, often sintered ceramic bodies, which do not satisfy the above-described requirements, have been manufactured.
A factor impairing the above-described required properties is presence of closed pores. Closed pores are closed spaces having major diameter of 0.5-2 mm in sintered ceramic bodies formed when they are prepared under some conditions.
The mechanism by which closed pores are produced in sintered ceramic bodies is considered to be as follows. During the manufacture of such sintered ceramic bodies, small particles of the organic binder are entrapped among the inorganic particles. This occurs because the ceramic slurry is prepared by dispersing the ceramic powder, a sintering promoter and the organic binder in water, and the organic binder remains during all the steps so that small organic particles remain in the formed compact. When the compact containing organic binder particles is sintered, the included binder combines with oxygen to form carbon dioxide gas. When heating of the compacts is started, sintering of the compact begins at a specific temperature and proceeds at a specific rate. If the sintering rate is greater than the rate of the reaction of the organic binder and oxygen, the sintering finishes before the formed carbon dioxide escapes out of the sintering compact and, as a result, closed pores are formed in the sintered ceramic body.
In order to expel the carbon dioxide out of the sintering compact before the sintering finishes by increasing rate of the carbon dioxide gas formation, the compact must be sintered at higher temperatures. This means that a more expensive apparatus, which withstands higher temperatures, must be used. Such is impracticable in view of the intention to manufacture spark plugs at lower cost.
Another cause of impairment of voltage withstanding ability, insulation property and mechanical strengths of the sintered ceramic body is presence of unavoidable impurities included in the raw materials. The sintering promoter is prepared by purifying clay. But it is impossible to completely remove impurities such as minute organic substance particles, fibers, etc. When a compact containing even a slight amount of unavoidable impurities is heated, they burn by the heat of sintering to generate a slight amount of carbon dioxide gas and minute voids are formed in the sintered ceramic body. It is considered that these minute voids also impair voltage withstanding ability, insulation property and mechanical strengths of the sintered ceramic body.
Sintered ceramic bodies to be incorporated in spark plugs are required to be manufactured at low cost in addition to having the above-described properties.
However, the alumina materials, which are conventionally used in manufacturing sintered ceramic bodies, contain a Na component, which exhibits high ionic conductivity, and therefore it is a matter of common sense among those skilled in the art to reduce the Na component content of alumina to not more than 0.05 wt % to satisfy the requirements for withstanding high voltage, good insulation property and high mechanical strengths of the resulting sintered ceramic bodies. As alumina raw material for the sintered ceramic body, low-soda alumina, which contains a Na content in an amount less than 0.1 wt %, is used by suitably purifying. This low-soda alumina is far more expensive than the medium-soda alumina, which is the Bayer Process alumina or the like and contains 0.1-0.2 wt % of Na component as Na.sub.2 O, and ordinary soda alumina, which contains not less than 0.2 wt % of Na component. As the alumina normally used to make spark plugs is obtained by further purifying low-soda alumina, which is already expensive, in order to reduce the Na content to a level of no more than 0.05 wt % as Na.sub.2 O, such conventionally used alumina for spark plug is highly expensive.
The use of the aforementioned medium-soda alumina in order to reduce the manufacturing cost of sintered ceramic bodies has received little attention in the art, because it is well known and obvious to those skilled in the art that the properties of the resultant sintered ceramic spark plug bodies, especially the properties of withstanding high voltage, mechanical strength and insulation property, are unsatisfactory.
The objects of this invention are to provide (1) inexpensive sintered ceramic bodies, which contain closed pores and minute voids far fewer than conventional sintered ceramic bodies and which have good voltage withstanding abilities, insulation properties and mechanical strength better than or of the same level as the conventional products, (2) a low-cost process for preparing such excellent sintered ceramic bodies, and (3) an inexpensive spark plug incorporating the sintered ceramic body having the above-described excellent properties.